The invention relates to a passive display device comprising a first and a second supporting plate at least one of which is transparent, a number of display elements for controlling the reflection or transmission of light each display element having at least one fixed electrode and an electrode which is movable with respect to said electrode by electrostatic forces and which is kept separated from the fixed electrode by means of at least one electrically insulating, oxidic layer.
A passive display device is to be understood to mean herein a display device of which the display elements themselves do not produce any light but reflect or transmit the ambient light in such a manner that a picture is obtained.
A passive display device of the above-mentioned electrostatic type is known, for example, from Netherlands Patent Application No. 7510103 in the name of the Applicants published on Mar. 1, 1977 and corresponding to U.S. Pat. No. 4,178,077, the published European Patent Application No. 85459 also in the name of the Applicants and "SID International Symposium Digest of technical papers", April, 1980, pp. 116-117. The movable electrode in each display element can be moved between two stable positions so that for light incident on the display device the transmission or reflection can be controlled per display element. The movable electrode is connected to one of the supporting plates by means of a number of resilient elements. The forces which urge the movable electrode from one stable position to the other are electrostatic forces whether or not combined with the resilient forces generated in the resilient elements.
In a first embodiment of the display device the movable electrode is moved between two fixed electrodes provided on the first and on the second supporting plate respectively. The resilient forces occurring in the resilient elements are usually negligible with respect to the electrostatic forces.
In a second embodiment of the display device the electrostatic forces urge the movable electrode from one stable position to the other and the resilient forces in the resilient elements are used to cause the electrode to return to its initial position. In both embodiments forming of a short-circuit between the movable electrode and a fixed electrode is prevented by an electrically insulating layer between said electrodes.
In its commonest form the first embodiment (also indicated by the term "three-electrode-system"), also comprises the second embodiment. In this commonest form the overall forces F.sub.t acting on the movable electrode may in fact be written as F.sub.t =F.sub.1 +F.sub.2 +F.sub.3, wherein F.sub.1 is the electrostatic force between the movable electrode and one fixed electrode; F.sub.2 is the electrostatic force between the movable electrode and the other fixed electrode, and F.sub.3 is the mechanical resilience generated in the resilient element. From the formula given for F.sub.t various embodiments of the display device may be derived. In the case in which F.sub.3 is negligibly small with respect to the terms F.sub.1 or F.sub.2 the movable electrode is moved substantially by means of electrostatic forces. In the case in which F.sub.1 or F.sub.2 is equal to zero, the above-indicated second embodiment is obtained.
The display device is suitable for operation in the reflection mode as well as in the transmission mode. When operating in the reflection mode the display device is filled with a liquid the color of which contrasts with the color of the surface of the movable electrode which faces the light incident on the display device. Dependent on the stable position the movable electrode is in, the display element in question will assume for the observer the color of the surface of the movable electrode or the color of the contrasting liquid. In this manner a picture can be built up by means of the picture elements.
When operating in the transmission mode, each display element forms a controllable light shutter. The construction then is, for example, such that the movable electrode comprises a pattern of light-pervious areas and that the fixed electrode on one of the supporting plates comprises a pattern of light transmitting areas which is the negative of that of the movable electrode. No light is transmitted if both electrodes are substantially in one plane.
In each embodiment an electrically insulating oxidic layer is provided between the movable electrode and the fixed electrode(s) as a result of which short-circuiting between the electrodes is prevented. The electrically insulating layer may be provided, for example, on the surface of the fixed electrode(s). The insulating layer may alternatively be provided on one or on both surfaces of the movable electrode or both on the fixed and on the movable electrodes. The electrically insulating oxidic layer is, for example, a layer of a metal oxide, for example TiO.sub.2. A very suitable and frequently used insulating layer is also a layer of SiO.sub.2 provided by means of a plasma CVD (Chemical Vapour Deposition) process.
When using the display device, for example a display device having a three-electrode-system, voltage pulses of +V and -V, respectively, are applied to the fixed electrodes, i.e. the fixed upper electrode and the fixed lower electrode, while a variable voltage pulse Vg is simultaneously applied to the movable electrode. If the voltage at the movable electrode is approximately -V the movable electrode will be repelled by the fixed lower electrode and be attracted by the fixed upper electrode. The movable electrode will then move adjacent to the fixed upper electrode. When a voltage of approximately +V is applied to the movable electrode, the movable electrode will move from the fixed upper electrodes to the fixed lower electrode.
Experiments performed by Applicants have demonstrated that when driving display elements in such a manner that the movable electrode would have to move from one stable position to the other stable position, such a movement sometimes does not occur or occurs only at a voltage applied to the movable electrode which is considerably larger than the theoretically required voltage.
In the non-prepublished previously filed Netherlands Patent Application No. 8402201 (PHN 11 103) in the name of the Applicants it is pointed out that the resistance experienced by the movable electrode when detaching from or approaching an engaging surface, i.e. insulating lager, is an important factor. It is stated more particularly that, upon detaching and approaching, the free space between the movable electrode and the engaging surface determines the value of the aerodynamic or hydrodynamic resistance to a considerable extent. It is suggested in the above-mentioned Netherlands Patent Application that the movable electrode and the engaging surface(s) be given different surface structures.